1. Field of the Invention
The present invention relates to an image-pickup display device that is a video see-through head mounted display (HMD) which is mounted on a head of a viewer and displays an image obtained by taking an external image to view it.
2. Description of the Related Art
Japanese Patent Laid-open No. H10-239628 and Japanese Patent No. 3604990 disclose the video see-through HMD as described above. The HMD includes a first prism that constitutes a displaying optical system which guides light from a display element displaying an original image to an exit pupil, i.e. the eye of a viewer, and a second prism that is disposed at an external side of the first prism and that reflects the light from the external to bend its optical path. The second prism forms an image-pickup optical system which is different from the displaying optical system, and guides the light from the external to the image-pickup element. Japanese Patent Laid-open No. H10-239628 discloses an example in which the second prism is joined on a surface at the external side of the first prism, and Japanese Patent No. 3604990 discloses an example in which the second prism is disposed at the external side of the first prism with a space.
In both the HMDs disclosed in Japanese Patent Laid-open No. H10-239628 and Japanese Patent No. 3604990, an optical axis of the image-pickup optical system is equal to a view axis of a viewer who looks into the displaying optical system. Therefore, the external image obtained by taking the external optical image using an image-pickup element is displayed on the display element as an original image so that the viewer can view substantially the same external image as that viewed by the naked eye via the displaying optical system. The external image and a computer graphics image are synthesized to be able to display them on the display element as if an object that does not really exist were in a space in front of the eye.
However, in each of the HMDs disclosed in Japanese Patent Laid-open No. H10-239628 and Japanese Patent 3604990, the displaying optical system and the image-pickup optical system are formed by completely different prisms, i.e. first and second prisms. Additionally, the first and second prisms have substantially the same size. Therefore, its weight increases as the size of the HMD is enlarged.
Especially, in the HMD disclosed in Japanese Patent No. 3604990, a position of the second prism is displaced with respect to the first prism due to a manufacturing error, a feeling of incongruity is given to the viewer because the displacement between the displayed external viewed via the displayed image and the real external or the position displacement of the CG image with respect to the displayed external becomes marked.